The invention relates to a method and an apparatus for monitoring the protective state of a cathodically protected structure connected to a protection circuit and at least one second electric circuit, whereby the protection circuit is periodically disconnected and the polarized potential of the structure is measured during the disconnected periods.
Buried structures, especially metal structures such as pipelines, will always corrode due to the galvanic activity at the interface between the surface of the pipe and the surrounding soil. In most cases, the corrosion rate can be reduced by displacing the electropotential towards lower values. Normally, this potential displacement is carried out by loading the object to be protected with a cathodic current so that a negative polarization is generated.
One of the problems associated with such cathodic protection systems is the ability to monitor and maintain the protection potential induced on the object.
In connection with the maintenance of cathodically protected structures it is well known to perform measurement of the protective state of the structures by periodically disconnecting the protection circuit with a time switch and by measuring the polarized potential of the structure during the disconnected periods.
This is known, for example, from U.S. Pat. No. 5,999,107, which describes a system for remotely measuring the state of a cathodically protected system by performing measurements and switchings in a control circuit by a wireless remote control. It is also normal to connect electric circuits in the form of decoupling capacitors and overvoltage absorbers to cathodically protected structures with the purpose of diverting induced alternating currents.
However, the polarized potential on a cathodically protected structure cannot be measured as long as decoupling capacitors or similar electric circuits are connected to the structure. It is for this reason that it is necessary to disconnect these electric circuits during the periods when the protective state is to be measured.
On extensive structures, such as gas and oil pipelines, it is very complicated and expensive to disconnect these electric circuits manually each time the polarized potential is to be measured and to reconnect them again after carrying out the measurements. The present invention seeks to overcome this problem is a new and useful manner.
The present invention provides a method and an apparatus for automatically disconnecting the decoupling capacitors or electric circuits containing the same during the periods when the protection circuit is periodically disconnected by the time switch.
The novel and unique features according to the invention, whereby this is achieved, is the fact that the voltage fluctuations which the periodic disconnections of the protection circuit generate on the cathodically protected structure are detected and that the detected signals are used for controlling the at least one second electric circuit.
Thereby, it is obtained that the maintenance work on extensive cathodically protected structures is facilitated considerably as the detected signals can be used directly for remote-controlling these functions or in replacement of manual work processes by replacing these processes by remote control.
In an especially advantageous method according to the invention, the detected signals are used via a breaker connected in series to the at least one second electric circuit for controlling this circuit. The electric circuit can be connected between the cathodically protected structure and an earthing system, and the signal can advantageously disconnect this connection when the polarized potential of the structure is to be measured.